Marine seismic explorations are usually carried out by a seismic cable fitted with several hydrophones being towed at a certain depth. Pressure waves are released near the cable in several ways. Usually this is carried out by means of air guns. The pressure wave energy travels down through the substratum, but parts of the pressure waves are reflected from areas where there are acoustic impedance characteristics in the substratum. The hydrophones record the reflected pressure waves in the water and transform this information into electric signals which are detected and processed on the seismic ship which tows the cable. Using this method, only reflected and/or converted shear to pressure energy is recorded. However, it is known that down in the substratum both pressure waves and shear waves are reflected. The shear waves do not travel in water and cannot therefore be detected by a hydrophone cable. Moreover, it is not possible with today's conventional hydrophone technology to detect the directions of the measured signals and this severely complicates possibilities for three-dimensional data recording.